Flattened lens structures in the form of a zone plate, Fresnel lens, or an aspherical lens are well established for a variety of optical applications and have been fabricated using a variety of materials. Such flattened lens structures can be made thinner than a comparable conventional lens allowing the construction of lenses of large aperture and short focal length without the mass and volume of material that would be required by a conventional lens design.
A Fresnel lens structure effectively divides the continuous surface of a standard lens into a set of surfaces of the same curvature with stepwise discontinuities therebetween. This allows the conventional lens structure to be made much flatter while otherwise functioning to focus light via refraction in a similar manner to a standard lens.
A zone plate is a related “flattened lens” type structure which uses diffraction rather than refraction to focus light and is generally in the form a series of concentric circular zones with alternating absorbing and transmitting zones. The focusing effect is created by constructive interference of waves passing through the transmitting zones.
An aspheric lens is a lens having a surface profile which is not a portion of a sphere or cylinder. An aspherical lens has a surface profile that can reduce or eliminate spherical aberration and also reduce other optical aberrations compared to a conventional lens. Furthermore, aspheric lenses allow a thinner, flatter lens to be made without compromising optical performance.
The material and geometry selected for such flattened lens structures will be dependent on the desired operating wavelength, focal length and thus the end application. In particular, a flattened lens structure will require a substrate material which is transparent at the operating wavelength and the zone structure of the flattened lens will have a geometry optimized to focus the light in the desired manner at the operating wavelength.
Traditionally, visible wavelength flattened lens structures have been fabricated in glass and plastic materials and have applications which vary from very large lens structures to very small lens structures including in lighthouses, book reading aids, projectors, and in camera optics and micro-optics.
For longer wavelength applications in the infrared region suitable infrared transparent substrate materials include CaF2, Si, Ge, ZnSe, and MgO. For example, M. C. Hutley et al. [“Blazed Zone Plates For The Infrared”, Proc. SPIE 0916, Infrared Systems—Design and Testing, 40, 1988] have reported a zone plate for focusing infrared radiation which is fabricated in germanium by ion etching, into the surface, a circular fringe pattern recorded in photoresist. An efficiency of 64% at 10.6 μm was measured. F. Gonzalez at el., [“Infrared antennas coupled to lithographic Fresnel zone plate lenses”, Applied Optics, vol. 43, no. 33, 2004] have also reported a zone plate for focusing infrared radiation based on a silicon substrate.
For longer wavelength applications UV transparent materials including quartz, CaF2, BaF2, and sapphire are widely used in optics. For example, Fresnel lenses in quartz are applied as beam homogenizers for a high power excimer and Nd:YAG lasers. Quartz is a suitable optical material at wavelengths ≥248 nm but for shorter wavelengths absorption of quartz strongly increases. Accordingly, CaF2, BaF2, and sapphire are more favourable optical materials in the UV region below 248 nm. G. Kopitkovas et al. [“Surface micromachining of UV transparent materials”, Thin Solid Films, 453-454 (2004) 31-35] have reported a surface micromachining method for such UV transparent materials which utilizes an XeCl excimer laser and an absorbing liquid in contact with the material for precise structuring of UV optical structures. In addition, D. Gil et al. [J. Vac. Sci. Technol. B 21(6) 2003] have reported the use of hydrogen silsesquioxane (HSQ) on a fused silica substrate with an absorber material such as chrome for fabricating diffractive optical elements that operate in the UV and deep UV regions.
For x-ray applications traditionally zones plates fabricated from high atomic number metals such as Ta, Au, Ir, and W have been used but it has been reported that the high heat load generated in the 2 to 12 keV photon energy range can deteriorate such materials. Wu et al. [Hard x-ray Zone Plates: Recent Progress, Materials 2012, 5, 1752-1773] have reported that the technology to focus hard x-rays (photon energy larger than 1-2 keV) made great progress in the period from 2009 to 2012 with the progress being particularly spectacular for lenses based on the Fresnel zone plate concept. It is reported that during this three year period the spatial resolution of such x-ray zone plates increased by a factor of three, opening up entirely new domains of application, specifically in biomedical research. It is also reported that diamond based zone plates have been developed which are stable at very high laser intensities citing work by Uhlén et al., David et al., and Wojcik et al. which is discussed in more detail below.
Uhlén et al. [“New diamond nanofabrication process for hard x-ray zone plates”, J. Vac. Sci. Technol. B 29, 06FG03 (2011)] have indicated that diamond is the optimal zone plate material for x-ray applications due to its high thermal conductivity and low absorption of hard x-rays. A number of masking and etching techniques are discussed for fabricating diffraction grating structures in diamond material. It is indicated that nanostructuring of diamond is primarily achieved via oxygen plasma based etching and that different combinations of hard masks and oxygen recipes have been used. Examples of hardmasks are disclosed as including HSQ, Ni—Ti, and Al. The authors report on a new tungsten-hardmask-based diamond dry-etch process for fabricating diamond zone plate lenses with a high aspect ratio. The tungsten hardmask was structured by electron-beam lithography, together with Cl2/O2 and SF6/O2 reactive ion etching in a trilayer resist-chromium-tungsten stack. The underlying diamond (dimensions 5 mm×5 mm×100 μm sourced from Diamond Materials GmbH) was then etched in an O2 plasma using the following parameters: 10 sccm O2 gas flow, 3 mTorr pressure, 100 W rf power, 200 W ICP, and 20° C. sample temperature. The authors report diamond gratings with half-pitch down to 80 nm and a height of 2.6 μm, as well as zone plates with a 75 μm diameter and 100 nm outermost zone width. The diffraction efficiency of the zone plates was measured to 14.5% at an 8 keV x-ray energy, and the imaging properties were investigated in a scanning microscope arrangement showing sub-100-nm resolution. Uhlén et al. suggest that the imaging and thermal properties of these lenses make them suitable for use with high-brightness x-ray free-electron laser sources. C. David et al. [Scientific Reports, Vol 1, Article no. 57, 2011] also disclose the fabrication of a diamond based Fresnel zone plate suitable for focussing of hard x-ray free electron laser pulses. Three types of zone plate were fabricated and tested by David et al. as discussed below.
The first type of zone plate fabricated by David et al. was a metal based zone plate rather than a diamond based zone plate and consisted of gold nanostructures. David et al. report that several lenses were irradiated with x-rays and it was found that the zone plates degraded rapidly and were completely destroyed within 3 minutes. The x-ray dose transferred to the gold structures per pulse was approximately 0.1 eV/atom which is below the dose required to initiate melting (0.4 eV/atom). However, due to poor heat dissipation, the temperature of the zone structures rose rapidly, triggering recrystallisation of the gold.
In order to improve the radiation hardness, a second type of zone plate was fabricated by David et al. based on diamond material. David et al. suggest that the excellent thermal conductivity, low x-ray absorption, and high melting point of diamond material render it ideally suited for this application. The diamond based zone plates were fabricated on polished 4-5 μm thick diamond membranes (Diamond Materials GmbH) supported by Si frames. The membranes were vapour-coated with a 5-nm thick Cr adhesive and conductive layer and then spin-coated with a 400-550 nm thick layer of negative-tone hydrogen silsesquioxane (HSQ) resist (FOx-16 solution, Dow Corning Corp). A zone plate pattern was formed in the HSQ via electron beam lithography and the Cr layer was removed by dry etching in a Cl2/CO2 plasma to expose the underlying diamond surface. The diamond layers were then etched in an inductively coupled plasma reactive ion etcher. It is reported that the parameters of the diamond etch which was used are similar to those described by Babinec, T. M. et al. [“A diamond nanowire single-photon source”, Nature Nanotechnology, 5, 195-199 (2010)].
Unlike the previously described gold based zone plates, the diamond zone plates were not damaged by x-ray exposure. However, David et al. indicate that at hard photon energies, the main draw-back of diamond is its low refractive index decrement at high photon energies, leading to very low diffraction efficiencies. In order to enhance the number of photons diffracted into the focal spot, a third type of zone plate was fabricated in which the diamond zone structures were filled with iridium by atomic layer deposition (ALD). This approach offers the strong phase-shifting property of this very dense, refractory material (density: 22.5 g/cm3, melting point: 2739 K) in intimate contact with a structure of interdigitated diamond cooling fins for optimised heat dissipation. David et al. report that while a pure diamond device was capable of diffracting only 2.1% of the incoming 8 keV radiation into focus, an Ir-filled diamond zone plate reached 13.2% efficiency at the same photon energy. Furthermore, it is reported that the diamond based device did not show any noticeable change in structural integrity or optical performance when applying the same pulse energy and pulse rate that destroyed the gold zone plates.
In contrast to David et al. who fabricated an Ir-filled diamond zone plate as described above, Wojcik et al. [J. Vac. Sci. Technol. B 28, C6P30 (2010)] have fabricated x-ray zone plates by depositing gold via electroplating onto an ultrananocrystalline diamond mold.
While the aforementioned documents have suggested the use of diamond material for x-ray zone plates, the zone plate structures are not suitable for applications at longer wavelengths in the infrared, visible, and ultraviolet regions of the electromagnetic spectrum. Furthermore, the present inventors have identified a number of potential problems with merely changing the geometry of the described structures to operate at longer wavelengths, particularly for high power optical applications in the infrared, visible, and ultraviolet regions. This is because while diamond materials are relatively transparent at x-ray wavelengths, only high quality optical grades of diamond material (e.g. as available from Element Six Ltd) have the desired transparency across the infrared, visible, and ultraviolet regions of the electromagnetic spectrum. Furthermore, even if high quality optical grades of diamond material are patterned to have an optical surface structure, achieving the desired optical performance whilst also providing a robust optical element with a high laser induced damage threshold in the infrared, visible, and ultraviolet regions of the electromagnetic spectrum is problematic. The optical performance of such surface finishes has been variable due to the difficulty in processing precisely defined surface patterns into diamond material because of the extreme hardness and low toughness of diamond materials. Furthermore, the processing methods required to form optical surface structures in diamond materials have resulted in significant surface and sub-surface crystal damage being incorporated into the diamond material. This surface and sub-surface damage in a synthetic diamond optical element causes a number of inter-related detrimental effects including: (1) a reduction in the laser induced damage threshold of the synthetic diamond optical element; (2) a reduction in the power at which the synthetic diamond optical element can operate; and (3) a reduction in the optical performance of the synthetic diamond optical element as a result of beam aberrations caused by the surface and sub-surface damage. As such it would be desirable to develop a process which forms precisely defined flattened lens structures in high quality optical grade synthetic diamond material without introducing surface and sub-surface crystal damage so as to achieve a synthetic diamond optical element which has a high laser induced damage threshold and high optical performance with minimal beam aberrations on transmission through the synthetic diamond optical element. Such synthetic diamond optical elements may be used at wavelengths in the infrared, visible, and ultraviolet regions of the electromagnetic spectrum for lens applications where optical performance and robustness to high laser powers is highly sensitive to the bulk crystal quality of the diamond material, the quality of the patterned optical surface finish, and the presence of any surface or sub-surface crystal damage caused during fabrication of the optical surface finish.
In light of the above, it is an aim of embodiments of the present invention to provide a synthetic diamond optical element comprising a flattened lens surface structure in the form of a zone plate, Fresnel lens, or aspherical lens formed directly in at least one surface of the synthetic diamond material and which is formed of a high optical quality synthetic diamond material while also having low surface and sub-surface crystal damage thus exhibiting a high laser induced damage threshold in the infrared, visible, and/or ultraviolet regions of the electromagnetic spectrum. It is a further aim to develop a technique for fabricating such flattened lens surface structures in synthetic diamond material which is relatively quick and low cost.